Combine pistachios, basil leaves, lemon juice, lemon zest, salt, Parmesan cheese, olive oil, and garlic clove (if desired) in a food processor.
Blend until smooth, scraping down the sides a couple of times.
If you want a looser pesto, feel free to add more olive oil.
Notes
Storage Tips: Refrigerate in an airtight container or jar for 3 to 5 days; press plastic wrap onto the surface or top with a thin layer of olive oil to slow browning.Freeze up to 3 months; freezing in an ice cube tray means you can take out what you need later.Expert Tips: Pistachios can vary in saltiness from brand to brand, so taste your pesto before you add the full amount of kosher salt and adjust from there.